Two stroke engines having longitudinal scavenging



March 15, 1955 o. PETERSEN TWO STROKE ENGINES HAVING LONGITUDINAL SCAVENGING Filed June '7, 1951 IN V EN TOR. an. M BY 2M MM TWO STROKE ENGINES HAVING LONGITUDINAL SCAVENGIN G Ove Petersen, Gentofte, Denmark, assignor to Aktieselskabet Burmeister & Wains Maskin-Og Skibsbyggeri, Copenhagen, Denmark Application June 7, 1951, Serial No. 230,270

Claims priority, application Denmark June 7, 1950 5 Claims. (Cl. 123-65) This invention relates to two-stroke engines of the type which operates with longitudinal scavenging, the scavenging air being through piston-controlled scavenging ports distributed substantially uniformly over the entire periphery of the cylinder, led into one end of the working cylinder, while the exhaust takes place through exhaust members at the opposite end of the working cylinder.

The essential characteristic feature of the invention consists in that a partition is provided within the usual scavenging air belt encircling the cylinder, which partition surrounds the scavenging port portion in such a manner as to define a space which surrounds the ports and is separated from the scavenging air belt, said partition being provided with non-return valves opening inwardly.

Hereby the disadvantages attached to all two-stroke engines having piston-controlled scavenging ports are overcome, which disadvantages are due to the fact that the top part of the piston necessarily must have a certain play in the cylinder, whereby gas from the cylinder will commence to blow out through the scavenging ports as soon as the top piston ring passes the upper edge of the scavenging ports during the expansion stroke. This blow-back of gas will involve an impurification of the scavenging air by combustion products, so that the fresh air led in for the next working stroke already beforehand has been somewhat diluted with combustion products. By the invention this blow-back of gas is considerably reduced because it may only now take place to the limited space just outside the ports, while the bulk air in the scavenging air belt is protected against impurification so that the charge remaining in the cylinder after the scavenging will consist of pure air. Hereby an increase of the maximum useful power and a better economy is obtained.

By the invention is also obtained an improved working condition of the engine. This is due partly to the fact that by the reduced and less intensive blow-back of gas through the scavenging ports a considerably smaller amount of cylinder oil will be forced out through said ports with the gas, and partly to the fact that the oil torn-out accumulates in the comparatively small space defined by the partition, the said space being easier to drain and keep free of waste oil than the entire scavenging air belt, to which the waste oil is not admitted now. This applies particularly to engines having advanced wear of the cylinder liner, where the piston rings have a tendency to leak, because the accumulation of oil in the scavenging air belt exactly in these cases involves serious problems.

This improved working condition of the engines involves also an essential reduced risk of waste oil fires in the scavenging air system. Such fires are due to the fact that blow-throughs past leaky piston rings may send flame jets into the scavenging air belt and ignite the waste oil there. Such fires involve serious troubles because the engine has to be stopped or be driven very slowly until the fire has been extinguished, and further the heating involves the risk of deformations of the cylinder liners and heat stresses in the environs of the cylinders, which may give rise to crackings. The risk of such fires has now practically been done away with, in part because flame jets cannot reach into the scavenging air belt proper, and in part because the amount of air available in the space surrounding the scavenging ports is too United States Patent 0 2,704,056 Patented Mar. 15, 1955 insignificant to feed a fire and finally because the said space as mentioned before may be kept almost free of waste oil.

In a preferred embodiment of the invention said partition is substantially circular and encircles the cylinder in the shape of a ring, the radius of which is only so much greater than that of the cylinder wall as to make the periphery adequate for accommodation a total area of non-return valves slightly larger-at least by 10% than the total scavenging port area. Hereby is obtained that the volume of the space defined round the scavenging ports becomes a minimum so that the blow-back of gas quickly stops due to the pressure equalization, and the impurified part of the scavenging air constitutes only a slight fraction of the total amount of scavenging air.

The non-return valves are expediently arranged in a ring carried out in two halves so that it can be bolted together about the cylinder liner whereby is obtained that the valves easily may be demounted and cleaned, the two halves of the ring being taken out through doors of sufficient size in the wall of the scavenging air chamber. The external diameter of the ring is adapted in such a manner that the provision thereof does not necessitate an increase of the cylinder distance beyond the normal distance, and that there nevertheless will be sufiicient space between two adjacent rings to allow the air to pass freely through the entire circumference of the said rings.

The invention also comprises an expedient design of the non-return valves proper, according to which the valves are constituted by elongated gaps in the said annular wall, parallel to the axis of the cylinder, and each covered internally by a leaf spring suspended on a spring system. This spring system may according to the invention be constituted by a hairpin-shaped leaf spring secured to the inner side of the ring between the scavenging ports.

This valve construction is distinguished by its extreme simplicity and cheapness, and offers only slight loss of pressure and operates practically without lag due to the insignificant weight of the movable parts.

The invention is illustrated on the accompanying drawing, on which Fig. 1 partially diagrammatically represents a multicylinder internal combustion engine embodying the invention,

Fig. 2 an enlarged, partial, horizontal section through a cylinder unit of this engine taken on the line 1111 of Fig. 3, and

Fig. 3 a vertical part section taken along the lines IIIIII in Figs. 1 and 2.

The engine as shown comprises a number of cylinders I mounted upon and projecting down into a common scavenging air belt 3 supported by and secured to a crank case 13 and bed plate 14 of optional design. Each cylinder 1 has a cylinder cover 12 with an outlet means 11 of conventional design.

Each cylinder has a circumferential row of scavenging air ports 2 situated within the scavenging air belt 3 and controlled in the ordinary manner by the working piston 15 with its sealing rings 16.

The piston is shown in its downward stroke in Fig. 3 in the moment when its upper edge lies flush with the upper edge of the scavenging air ports 2 when the piston should theoretically commence to uncover the scavenging air ports. As illustrated, in an exaggerated manner, in Fig. 3, the exposure of the scavenging air ports has however commenced already some time before, viz, in the moment when the upper edge of the upper piston ring 16 passes the upper edge of the scavenging air port in the downstroke. This is due to the fact that the piston crown 17 in order to allow for heat expansion and to avoid piston seizure must have a radial clearance 18 in the cylinder. Through the annular space thus existing between the piston crown and the cylinder wall connection will be established from the cylinder working space to the scavenging ports 2 slightly before the piston arrives in the position shown in Fig. 3 and thus slightly before the cylinder pressure has been completely relieged through the exhaust means in the upper cylinder en About the scavenging port portion of each cylinder is now in the scavenging air belt placed an annular partition 4, which defines a narrow annular space surrounding the scavenging ports and separates it from the remaining volume of the scavenging air belt. This ring is as shown in Fig. 2 expediently made in two halves which are assembled with a flange joint 10, and which may be mounted and demounted one by one through a suitable opening 8 in the outer wall of the scavenging air chamber. This opening is normally closed by a bolted-on cover 9.

Opposite the spaces between the scavenging ports 2 vertical rectangular gaps are provided in the annular wall 4, which on the innerside are covered by flat leaf springs 6, which can lie sealing against the gaps but are pressed backwards by an external over pressure. The dimensions are so chosen that the area of flow procured hereby for flow from the scavenging air belt into the annular space is at least 10% greater than the total sectional area of the scavenging ports 2, whereby is obtained that the loss in pressure by the flow through the gaps remains insignificant.

The leaf spring 6 is by its top end suspended on a spring system consisting of a hairpin-shaped leaf spring 7, which is bolted on to the wall of the ring above the scavenging ports and extends down between the latter. For the sake of simplicity only one spring device or unit is shown in Fig. 2.

It appears directly from the figure that a blowback of gas from the interior of the cylinder through the scavenging ports will press the leaf springs 6 tightly against the innerside of the annular wall 4 thereby preventing exit through the gaps 5. In the narrow space inside the annular wall a quick equalization of pressure with the cylinder will then take place in such a manner that the blowback of gas is highly limited, and the entire bulk of scavenging air in the receiver is kept free from impurification with the gas blown back from the cylinder. As soon as the pressure in the cylinder has dropped below the scavenging pressure, i. e. generally while at the same time when the upper edge of the piston in descending passes the scavenging ports, the scavenging commences in normal way during which the over pressure in the scavenging air belt keeps the non-return valves 6 open.

The invention is not limited to the shown and described embodiment, the details of which may be varied in various ways within the scope of the invention. Thus it is expedient but not strictly necessary that the space surrounding the scavenging ports and defined by the partition with the non-return valves shall be of less height than the scavenging air belt, so that the latter extends over or/and under the said space, as hereby free communication between the parts of the scavenging air chamber lying on either side of the cylinder row and consequently a uniform distribution of the scavenging air are obtained. Other constructions of non-return valves than those shown may be used, the most essential feature being that the movable parts of the valve are light so as to work without essential lag and without causing any essential loss in pressure. It is conceivable to place the spring devices of the valves outside the partition surrounding the cylinder, whereby the latter may be placed still closer to the cylinder, and the space limited inside the partition can be made correspondingly smaller 4 in volume. The valve body proper may in that case be made of a light plate supported on arms extending through the gap or similar opening in the partition controlled by the valve. The valve bodies may also be placed so as to pivot about a virtual or a real axis during their opening and closing motion, and preferably in such a manner that the air by the inlet through the nonreturn valves will be made to rotate in the same direction as the possibly oblique direction of the scavenging engine having a scavenging air supply belt, a working cylinder extending down into said belt and having a piston working therein, a peripheral row of substantially uniformly distributed scavenging air ports in the wall of said cylinder within said belt, exhaust means in the opposite end of said cylinder working space, said scavenging ports being arranged in the vicinity of the outer dead center position of the working piston so as to be uncovered thereby with such delay after the opening of said exhaust means as to allow the cylinder pressure to drop substantially to the pressure prevailing in the scavenging air belt before the piston clears said ports, and a partition within said belt for said cylinder and surrounding the cylinder 'in radially spaced relation thereto and enclosing the scavenging port portion thereof to form a confined space around said ports separated from the interior of the said belt, said partition having a wall substantially concentric to the cylinder and nonreturn valves opening towards the cylinder being provided in said wall of the partition.

2. In a multicylinder two-stroke internal combustion engine as in claim 1 in which said partition is made in two halves adapted to be assembled about the cylinder mer.

3. In a multicylinder two-stroke internal combustion engine as in claim 1 in which said non-return valves comprise elongated slots in said wall parallel to the axis of the pertaining cylinder and each provided on the inner side of the said partition with a valve body consisting of a leaf spring suspended on a spring suspension.

4. In a multicylinder two-stroke internal combustion engine as in claim 3 in which the said spring suspension comprises a hairpin-shaped leaf spring secured to the inner face of said partition in staggered relation to the location of the scavenging air ports in the cylinder wall.

5. In a multicylinder two-stroke internal combustion engine as in claim 3 in which said spring suspension is mounted on the side of said partition remote from the cylinder.

References Cited in thefilc of this patent UNITED STATES PATENTS 1,092,109 Kempton Mar. 31, 1914 1,168,331 Renter Jan. 18, 1916 1,407,585 Setz Feb. 21, 1922 1,407,586 Setz Feb. 21, 1922 1,672,436 Thege June 5, 1928 1,992,328 Schaer Feb. 26, 1935 FOREIGN PATENTS 233,246 Great Britain May 7, 1925 60,559 Netherlands Feb. 16, 1948 

